SB 741 
.03 H3 
Copy 1 



SB 741 
.03 H3 
Copy 1 



ED STATES DEPARTMENT OF AGRICULTURE 



^ 



f BULLETIN No. 453 vv^™ ,« 

t^^ Contribution from the Bureau of Plant Industry <r* VI kX-x^ f/P^ 

J^^^ WM. A. TAYLOR, Chief ^^^^Sl 



Washington, D. C. 



PROFESSIONAL PAPER 



January 20, 1917 



THE CONTROL OF DAMPING-OFF OF CONIFEROUS 

SEEDLINGS. 

By ('arl Hartley, Forest Pathologist , and Roy G. Pierce, Forest Assistant, Office of 
Investigations in Forest Pathology. 



CONTENTS. 

Page. 



The disease 

Economic importance of damping-off 

Relation between nursery metliods and the 

control of damping-off 

Tests of soil disinfection for the control of 

damping-off 

Soil-disinfection tests summarized 



Cost of disinfectant treatments 

Secondary advantages from disinfectant treat- 
ments 

Conclusions as to soil disinfectants 

Soil treatments recommended 

Summary 



Page. 
19 

21 
24 
28 
31 



THE DISEASE. 



Damping-off is a term commonly used to describe the disease caus- 
ing the death of very young seedlings due to various parasitic fungi. 
The damping-off of conifers in the United States has been found to 
be caused by the fungi Pi/thium debar yanum Hesse, Fusarium mo- 
niliforme Sheldon, and the common American Rhizoctonia, usually 
referred to as Corticium vagum B. and C. var. solani Burt. Other 
species of Fusarium and other fungi are probably also concerned to 
a less extent. Most affected seedlhigs promptly fall over and decay. 
Those which are not attacked till they are 4 to 8 weeks old may have 
such wiry stems that they merely turn brown and remain standing 
pffrr death. This later type of injury is caused by parasites in the 
tine way as the decay of the more succulent younger seedlmgs, 
though nurserjrmen do not always recognize it as the same thing. 

ECONOMIC IMPORTANCE OF DAMPING-OFF. 

Damping-off has been a handicap to nearly all nurserymen who 
raise conifers from seed. In most nurseries a large number of seed- 
lings are lost every year. The loss is ordinarily considerably heavier 
than the nurseryman realizes. Very young seedlings decay and dis- 
appear so soon after infection that the number of dead seedlings 

(30142°— Bull. 453—17 1 

Monograph. 



Cir 



*3 



? 



■Us //j 

2 BULLETIN 453^ U. S. DEPAETMENT OF AGRICULTURE. 

visible at any one time is but a small part of the total loss. Fur- 
thermore, many of the seedlings are killed immediately after the 
seed sprouts and before the seedlings appear above the soil surface. 
Many failures hitherto attributed to poor germination are in reality 
due to the work of the damping-off parasites in the sprouting seed, 
underground. The high price of most evergreen seed makes this loss 
of young seedlings a serious matter. 

The cost of seed ' ranges from a minimmn price of 50 cents per 
pound for western yellow pine (Firms ponderosa Laws.) collected by 
the United States Forest Service to $2 to S4 per pound quoted by 
commercial seedsmen for the native spruces and $5 to $10 per pound 
for Norway pine (Pinus resinosa Ait.). The regular annual loss is, 
however, not the most serious result of the disease. The most trouble- 
some thing from the commercial standpoint is the great variation in 
the prevalence of the disease. In some seasons the loss is relatively 
slight, while in others the seed beds may be almost a total loss. The 
results of a damping-off epidemic are shown by the poor stand in 
the untreated plats in Plate I. Such epidemics make it impossible 
for a nurseryman to secure any regularity in production. The diffi- 
culty of controlling damphig-off has caused many nurserymen to 
give up raising their own seedling conifers. 

The loss from damping-off can not be figured merely on a basis of 
the number of the seedlings destroyed. The most serious aspect of 
the disease is the extent to which planting is discouraged by it. Re- 
forestation of watersheds is one of the great needs of the present time. 
When interest on the cost of a forest plantation is compounded for 
the 80 or 100 years which must elapse between forest planting and 
tunber cutting, a very slight initial increase in the cost of planting 
stock becomes a heavy charge against the ultimate timber value of 
the plantation, on which the owner must depend for direct returns. 
It is necessary in order to encourage forest planting to eliminate aU 
possible cost items in the establishment of plantations. Both the 
average loss and the irregularity in production due to damping-off 
are reflected in the prices of coniferous planting stock, so that the 
disease must be controlled to give the maxunum opportunity for 
profitable reforestation. It has previously been possible to import 
pine stock from Europe cheaply. This has resulted in the introduc- 
tion of very dangerous insect and fungous pests. The importation of 
all pines is now prohibited, and the danger of the further introduction 
of parasites on imported stock of other conifers makes it necessary 
that the domestic nursery industry be developed on an economic 
scale that wiU eliminate all need or incentive for importation. To 
do this, all important native diseases, of which damping-off is the 
most serious, must be controlled. 

1 Information furnished hj the Office of Forest Investigations, United States Forest Servict. 

D. of: D. 

JAN 25 1917 



DAMPING-OFP OF CONIFEROUS SEEDLINGS. 3 

RELATION BETWEEN NURSERY METHODS AND THE CONTROL OF 

DAMPING-OFF. 

A great deal has been written, largely in horticultural journals, on 
the methods best calculated to prevent damping-off. It has been 
generally agreed that beds should be well drained and aerated and 
should have no more water or shade than absolutely necessary. Sur- 
facing beds with gravel, coarse sand, or heated sand has also been 
recommended. All of these measures have value, but all have their 
limitations. Ml of them combined are not sufficient to prevent 
heavy losses when the damping-off parasites are present in sufficient 
quantity. 

THE BEST SOIL FOR SEED BEDS. 

In general, where there is a chance to choose between different seed- 
bed sites it is safest from the standpoint of damping-off control £o 
select a site with a sandy soil. Either because such soils are better 
dramed or because they contain less organic food matter for fungi, 
damping-off is commonly less troublesome on light than on heavy 
soils. A sandy soil is by no means a guaranty of freedom from 
disease, the heaviest damping-off losses in the writers' experience 
having occurred on a soil consisting of nearly pure sand. Alkaline 
soils are believed to favor damping-off. If this is found to be the case, 
it may explain the fact that some of the heaviest losses from damping- 
off occur in Nebraska and Kansas rather than in the more humid 
Eastern States. 

Wliere it is necessary to put seed beds on a soil where damping-off 
is troublesome, the soil may be improved in various ways. Where 
sand is easily available, it may pay to haul it in and mix it with the 
surface soil. Excellent results have been obtained in one case in the 
writers' experience by making the entire uppor 3 inches of the bed 
of very sandy subsoil just dug up. A less expensive procedure which 
has also given indication of value is to cover the seed with subsoil 
taken from a point so far below the surface that it is likely to be free 
from parasitic fungi and an imfavorable medium for their growth. 
It is usually better to cover seed with sand than with heavy soil, and 
sm-facing the beds with coarse sand or gravel after the seed is sown 
and covered is considered helpful. Surfacing the beds after germi- 
nation with heated sand applied as hot as the hand can stand has also 
been recommended. Tests by the writers of hot sand on seed beds 
in a sandy western soil had no perceptible effect on the disease. 

AVOIDANCE OF EXCESSIVE MOISTURE. 

To secure drainage, seed beds are commonly raised from 2 to 3 
inches above the paths, and at some nurseries the surface of the bed 
is arched to increase run-off. This is probably good practice at most 
nurseries, but on very sandy soils in a dry climate it does not appear to 



4 BULLETIN 453, U. S. DEPARTMENT OF AGRICULTURE. 

have any value whatever. To secure aeration the best eastern 
nurseries leave their beds entirely open to the wind, or, if side walls 
are needed to exclude seed-eating animals, they use wire netting 
only. Yet, at a nursery in western Kansas, after trying the rather 
expensive netting sides, it was found by forest officers that as good 
or better results were obtained by using beds with tight board sides. 
It is well known that excessive moisture and shade must be avoided 
in the seed beds. However, in sandy soils or in a dry country there 
is nearly as much danger of getting the beds too dry as of keeping 
them too wet. Drought often kills large numbers of seedlings in 
beds left without water. 

Aside from common drought injury the effort to keep seed beds dry 
may result in an entirely different type of trouble. If seed beds are 
insufficiently shaded and the soil surface is not kept moist, there is 
at some nurseries serious loss from ''white-spot" injury. This injury 
in seedlings up to 3 weeks of age appears as a whitening and shrink- 
ing of the stem just above the ground line. The shrunken area in 
most cases girdles the stem, the seedling falls over, and death follows. 
When the injury occurs on one side only it is usually the south or 
southwest side of the stem that is affected. The symptoms are very 
much like damping-off, the decided white color of the part of the 
stem first affected being the most noticeable difference. This white- 
spot injury is caused not by j)arasites but apparently by heat, with 
which possibly may also be combined the direct effects of intense 
light. The temperature at the surface of a dry, loose soil exposed to 
the sun may exceed 140° F., a very high temperature for most tender 
plant tissues to endure for any length of time. White-spot injury is 
almost always mistaken for damping-off, and has been observed to 
kill more than half of the seedlings in unshaded beds of even so heat- 
resistant a species as western yellow pine (Pinus i^onderosa) . In 
withholding water and shade from coniferous seed beds, nurserymen 
must be very careful or the loss from drought and white-spot will 
much exceed the advantage from decreased damping-off loss. Keep- 
ing the beds dry is impossible in wet weather and dangerous in dry 
weather, and it therefore will never be more than a partial control 
method for damping-off. 

FERTILIZERS. 

The relation between soil fertilization and damping-off has been 
but little investigated. In preliminary tests dried blood and nitrate 
of soda have apparently favored the damping-off parasites, and their 
use on pine seed beds is considered inadvisable. However, in Ver- 
mont* tankage has given excellent results against damping-off. It 
is evident that further tests of nitrogenous fertilizers are needed. 

1 Gifford, C. M. The damping-off of coniferous seedlings. Vt. Agr. Exp. Sta. Bui. 157, p. 170. 1911. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 5 

Lime in small (juiuititics lias had no a])pr('(iablo effect, while applica- 
tions at the rate of 1 ton per acre to beds of jack pine (Pinus hanhn- 
ana Lamb.) in Nebraska have had bad effects. At Burlington, Vt.,^ 
wood ashes have been added to seed beds with disastrous results. 
Lime also caused trouble at Burlington. At Providence, R. I., 
a mixture of coal and wood ashes tested by Prof. H. H. York 
seemed to give bad results. Heavy applications of imperfectly rotted 
horse manure have been known to greatly increase damping-off at 
the Nebraska nursery, but at the same nursery well-composted 
manure seems harmless. It is probably safest to manure beds at 
least a year before seed is to be sown on them. No trouble has been 
observed to result from the use of green manures, but it may be 
found that some such crops harbor damping-off parasites and should 
be avoided. The perennial ragweed of the Southwest (Amhrosia 
psilostachya DC.) serves as a hold-over host for parasitic strains of 
Corticium, so that this and any other weeds kno^vn to carry seedling 
parasites should be eliminated from nursery areas. 

DENSITY OF SOWING. 

As with truck crops, it is found that pines suffer most from damping- 
off when sown too closely. A bed containing a stand of seedUngs 
which is too dense will not only lose more seedlings than a less 
crowded bed, but it will lose a higher percentage of its seedlings. 
This is due to the ease with which the parasites spread from one 
seedling to another in dense stands. It has also been found with 
jack pine that in tests at two nurseries the damping-off loss in seed- 
lings sown broadcast was only four-fifths as great as in adjacent 
plats sown in drills. In general, beds sown broadcast seem to suffer 
less from damping-off than beds sown in drills, though with western 
yellow pine broadcasting has given no better results than the driU 
method at the nurseries where comparative tests were made. 

TIME OF SOWING. 

Another item in nursery practice in which variation may affect 
damping-off is the time at which seed is sown. At some nurseries 
it makes little difference when seed is sown. In one season beds 
sown in early spring suffer least from the disease. The next season 
the latest sown beds may come out the best. However, at some 
nurseries it is found that there is a best time and a worst time for 
seed sowing. In a New Mexico nursery it has been demonstrated 
that July, the local rainy season, is the worst sowing time from the 
damping-off standpoint. At two western nurseries (in Nebraska and 
Colorado) the Forest Service has found that yellow-pine seed beds 
sown in late autumn are comparatively free from damping-off. 
Repeated tests during two or three successive seasons are necessary 

> Gifford, C. M. Op. cit. 



6 BULLETIN 453, V. S. DEPAETMENT OF AGRICULTURE. 

to determine for any particular nursery whether or not damping-off 
losses are regularly less in beds sown at a particular time. It is 
thought that fall sowing is least likely to succeed in locaUties in 
which the soil does not remain constantly frozen during the winter. 

TESTS OF SOIL DISINFECTION FOR THE CONTROL OF DAMPING-OFF. 

The entire matter of the factors controlling the work of the 
damping-off parasites and the methods of seed-bed management 
most likely to decrease the disease needs a great deal of further 
examination. At present adherence to the best known nursery 
practice will not avoid considerable annual losses at most nurseries 
or prevent epidemic years in which the beds of certain species are 
entire failures. The multiplicity of parasites and the different con- 
ditions of soil and climate to be met so comphcate the problem that 
it has been found most profitable to make a direct attack on the 
parasites by the use of disinfectants rather than wait for results by 
the indirect means of changed nursery management. 

Experiments in seed-bed disinfection have been carried on by the 
writers or in pursuance of their recommendations for the past seven 
years and at a number of nurseries. At different times assistance 
has been rendered by Mr. R. D. Rands, Dr. J. V. Hofmann, Dr. T. C. 
Merrill, Mr. S. C. Bruner, and Mr. G. G. Halm. 

Definite and satisfactory control of the disease has been secured 
by soil disinfection at every nursery in which extensive experiments 
have been conducted, and preliminary tests at additional nurseries 
indicate that while different places require somewhat different pro- 
cedure, damping-off everywhere can be controlled by proper disin- 
fection of the seed beds. The economic results at all of these nurs- 
eries are briefly described in the following pages, together with a 
summary of the published results of other investigators. 

RESULTS OF SOIL DISINFECTION AT NURSERIES WHERE REPEATED TESTS HAVE BEEN 

MADE. 

The first tests were made in pine seed beds in the very sandy soil of 
the United States Forest Service nursery at Halsey, Nebr. The ex- 
perience at this nursery was in many ways typical and will therefore 
be described in some detail. 

The first attempt was to control the disease by applying disin- 
fectants after the seedlings appeared above gromid. This failed, 
partly because the disinfectants injured the seedlings and partly be- 
cause the parasites were found to do a great deal of their work before 
the seedlings came up. A number of disinfectants were then tested in 
applications at or before seed sowing. Of these, commercial sul23huric 
acid appeared the cheapest and most effective. Both heat and formal- 
dehyde, the means usually recommended for disinfecting greenhouse 
and truck soils, proved less reliable as well as more expensive. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 7 

Tli(^ treatment finally adopted consists of tlu-ee-sixtoenths fluid 
ounce of sulphuric acid per square foot of seed bed, applied in solu- 
tion immediately after the seed is sown and covered. The amount of 
water used to dissolve the acid varies from 1 pint per square foot 
when the soil is wet to 2 pints when the soil is dry. In all disinfec- 
tion of seed beds by chemicals the quantity of the disinfectant used 
per unit area of soil surface seems to be the important variable. The 
disinfectant must be dissolved in sufficient water to permit its dis- 
tribution tlu-ough the soil to a depth of several inches, but within 
certain limits the concentration of the solution as applied does not 
appear to be an important factor. The adherence of some investi- 
gators to the concentration of the solution used as a measure of soil 
treatment makes their work difficult to correlate. 

In the earlier tests of acid at Halsey chemical injury to the pines 
occurred m beds treated with acid at the time of sowino;. It was 
found that dormant pine seeds were not mjm'ed by acid, but that the 
root tips of the seecUings in acid beds were often killed just after 
germination commenced. The way in which this injury occui-red 
made it evident that it was due to the concentration of the acid in 
the surface soil. Evaporation of water from the soil surface seemed 
to bring up the dismfectant from the lower soil, just as alkah salts 
come to the surface in alkali soils. Watering the beds frequently 
during the germinating period prevents such surface concentration 
and m practice has been found entirely to prevent injury to the seed- 
lings. The method of protecting the seedhngs from acid injury at 
this nursery is to water the seed beds frequently from the time seed 
is sown until a few days after germination, when the root tips have 
penetrated one-half inch mto the soil. After this time they are prac- 
tically safe from further injuiy. The beds are w^atered daily in ordi- 
nary spring weather, every other day in misty or rainy weather, and 
twice daily when the maximum temperature exceeds 80° F. In 
clear weather each watering is at the rate of approximately 1 ^ pints 
per square foot of bed. In cold, cloudy weather 1 pint per square 
foot is found sufficient. (Two-tenths inch of rainfall is equivalent 
to 1 pint per square foot.) 

The acid treatment was repeatedly tested on cUfferent species of 
pine during different years and at different times of the year. In all 
cases it resulted in increased stands. The results of all the acid tests 
in which the above watering system or its equivalent was followed 
are given in Table I. 

In addition to the three species given in this table, a test on a 
series of plats of Corsican pine (Pinus laricio Poir.) gave excellent 
results; an accident prevented the securing of exact figures. Some 
of the tests included in the table were conducted on a considerable 
scale, the total area involved being 8,350 square feet in the treated 



8 BULLETIN 453, U. S. DEPAETMENT OF AGRICULTURE. 

plats and 547 feet in the untreated [)lats. In the case of the larger 
plats, counts of seedlings were made only on sample areas scattered 
through the plats. The percentages of germination and of death 
are based on counts of all seedlings on 246 square feet in the treated 
plats and 145 square feet in the untreated plats. Tlie surviving 
seedlings were counted on these sample areas and also on additional 
areas, totaling 406 square feet for the acid plats and 196 square feet 
for the untreated plats. Damping-off is so variable that only 
repeated and extensive tests- of this sort are entirely reliable. 

Table I . — Control of damping-off of pines by three-sixteenths fluid ounce of sulphuric acid 
per square foot of seed bed applied at sowing time, Halsey, Nebr. 





Number 
of sepa- 
rate tests. 


Treat- 
ment. 


Absolute results. 


Relative results. 


Species. 


fiermina- 
tion.i 


Death 
per hun- 
dred seed- 
lings. 2 


Final 

staTid 

based on 

seedsown. 


Germina- 
tion. 


Death. 


Final 
. stand. 


Pinus banksiana 3 


n 

3 
3 

17 


Arid 

None 

Aoid 

None 

Acid 

None 

Acid.... 
None 


Per cent. 
32.7 
20.3 
66.3 
50.3 
70.0 
62.7 


15.2 
43.7 
12.0 
36.7 
3S.7 
73.7 


Per cent. 
26.4 
9.9 
58.7 
33.0 
40.7 
16.0 


161 
100 
1.32 
100 
112 
100 


35 
100 

33 
100 

53 
100 


267 
100 
178 


P. resinosa. 


100 
254 




100 


Average ^ 


56.3 

44.4 


22.0 
51.4 


41.9 
16.3 


135 
100 


43 
100 


233 
100 



1 The germination prrcentage takes into account only seedlings which appear above the soil. Seeds 
which started to germinate but did not reach the point of breaking through tlio soil are not included. 

2 The death percentage inckides all seedlings whicli died from damping-otf and also any which may have 
died from drought or chemical injury after the seedlings came up. Seedlings broken by hail or other 
mechanical means or killed by insects are not included. 

3 Germination and death figures obtained from only 9 of tne 11 jack-pine tests. 

The acid in practically all cases caused a marked increase in the 
number of seedlings that came up, as well as a decrease in the death 
rate after coming up. Damping-off was not entirely controlled. 
There was still a loss of 22 per cent after germination in the treated 
plats, but the loss in the untreated plats was nearly two and one-half 
times as great. The combined effect of increased germination and 
decreased death rate was a large increase in the number of healthy 
seedlings produced. (See Table I, columns headed ''Final stand.") 

It is especially to be noted that in jack pine and Norway pine, 
the species with which the most extensive tests were made, more 
than 250 healthy seedlings were obtained in the treated plats for 
every 100 obtained from equal quantities of seed in the untreated 
plats. The results of the acid treatment of Norway pine are strik- 
ingly shown by the difference in stand in the treated and untreated 
plats shown in Plate I, figure 1. 

Since the foregoing tests were conducted, the acid treatment has 
been put into regular use on all spring-sown beds at this nursery and 
the success of the treatment further confirmed by the continued 
good results secured. • 



Bui. 453, U. S. Dept. of Agriculture. 



Plate I. 




Fig. 1.— Norway-Pine Seed Bed, 1 Year Old, Halsey, Nebr. 

Plat in center untreated Remainder of bed treated at time of sowing with a solution containing 
three-sixteenths fluid ounce of sulphuric acid per square foot of bed. Practically all of the seedlings 
in the untreated plat have been destroyed l:)y damping-ofl. Photographed by Charles B. Pool 
(Fl). 




Fig. 2.— Western Yellow-Pine Seed Bed, Kansas Sandhills. 

Foregromid untreated. Background treated at time of sowing with a sulphuric-acid solution. Jfost 
of the seedlmgs in the untreated plat have been destroyed by damping-ofl. Photographed by Dt, 
J.V. Hofmann(F2). 



Bui. 453, U. S. Dept. of Agriculture. 



Plate II. 




Fig. 1.— Jack-Pine Seed Bed, Garden City, Kans. 



Foreeround untreated. Background treated with sulphiu-ic-acid solution before germination. 
Stakes marked in inches. Note the difEerence in height of the seedlings after the first season s 
growth (F 3). 




FiQ. 2— Jack-Pine Seed Bed, Kansas Sandhills. 



Plat at left treated with one-fourth ounce and plat at center ^ith three-eighths ounce of sulphuric 
acid per square foot. Plat at right untreated. Note the relative ^eedom from weeds in the 
treated plats. The apparently thin stand of trees in the treated plats is due *» t^® Vf y ^^Jf 'I'f^ 
of the young seedlings of this species of pine. Actual counts.in these plats showed the stand to 
be as dense as can be grown without subsequent overcrowding. Photographed by Stephen u. 
Bruner (F 4). 



DAMPIISTG-OFF OF CONIFEROUS SEEDLINGS. 9 

At nearly all of the other nurseries sulphuric acid was also found 
successful in controlling damping-off. However, because of dif- 
ferences ill soil and climate, the treatments required elsewhere dif- 
fered in some details from that at Halsey. On a still lighter sand 
in Kansas sand liiUs, which was probably also somewhat more alka- 
line, it was found that damping-off could be controlled best by a 
heavier treatment of acid — one-fourth fluid ounce per square foot in 
ordinary spring-sown beds, and five-sixteenths or three-eighths ounce 
in beds sown in the fall or very early spring. Tlie watering required 
during the germinating period to prevent injury to the pines was 
less than at Halsey. The losses in untreated beds at this nursery were 
exceptionally heavy. In the treated beds it was not possible to 
reduce the damping-off loss to as low a figure as at Halsey, probably 
because of the common occurrence of Fusarium. moniUforme, a fungus 
exceptionally able to quickly reinfect treated beds. The success of 
the treatment from the economic standpoint, however, was greater 
than at Halsey. Without the treatment, failure occurred in the 
seed beds more often than success, and stock could not be raised at 
a reasonable expense. With the treatment, success was the rule, 
and the economical production of stock became possible. The dif- 
ference in stand between treated and untreated plats at this nursery 
is shown in Plate I, figure 2. 

At a single nursery on a fine sandy soil near Morrisville, Pa., it was 
found that it was more difficult than at Halsey to prevent acid injiu-y 
to germinating seedlings. This was a nursery at which tests were 
made during a single season only. It is mentioned at this point merely 
to show the difference in behavior of acid in different soils. Even one- 
twelfth fluid ounce per square foot caused injury to the pines at Mor- 
risville. Indications from the small series of tests conducted there 
were that one-eighth ounce of acid per square foot would probably 
prove effective against damping-off and harmless to coniferous seed- 
lings if followed by sufficiently frequent watering during the germi- 
nating period. It is likely that this soil was slightly acid to start 
with. 

In marked contrast to the experience at Morrisville are the results 
obtained at Fort Bayard, N. Mex., Monument, Colo., and Haugan, 
Mont. At all of these places acid at the rate of three-sixteenths ounce 
or more per square foot has been applied to the beds at sowing in 
repeated tests, without any injury to the seedlings that could be 
detected by the forest officers in charge at the nurseries. At Fort 
Bayard Mr. H. C. Turner has tested the acid very thoroughly for four 
successive seasons and in quantities up to five-eighths ounce per square 
foot without finding any injury to seedlings. Even three-sixteenths 
ounce proved reasonably efficient in decreasing damping-off, and the 
five-eighths ounce treatment reduced the loss to less than 1 per cent. 

60142°— Bull. 453—17 2 



10 



BULLETIN 453, U. S. DEPARTMENT OF AGRICULTURE. 



At Haugan the acid has been used for three years and damping-olf 
has been practically abolished in the beds treated. These theee 
nurseries differ from the preceding three in that the soil is heavier 
and there is no need of any extra watering in order to prevent chem- 
ical injury to the seedlings. 

At a seventh nursery an entirely different condition was encoun- 
tered. The seed beds were located on a rather heavy soil near 
Garden City, Kans. This soil effervesced vigorously when treated 
with acid, yielding 0.23 per cent of carbon dioxid from the surface 
soil and over 1 per cent from the subsoil in a test made by the Bureau 
of Soils. Acid treatments had no effect on damping-off in this soil, 
owing presumably to the high carbonate content and consequent 
alkalinity which the effervescence indicated. Tests with copper 
sulphate, one-fourth ounce per square foot, and zinc chlorid, one-half 
ounce per square foot, showed that both these and heavier treatments 
were effective against damping-off. The results are given in Table IL 

Table II. — Control of dam-ping-off of pines by one-fo\irth to three fourths ounce copper 
sulphate and one-half to one ounce zinc chlorid per square foot of seed bed, Garden City, 
Kans. 





Disinfectant. 


Num- 
ber of 
seiia- 
rate 
tests. 


Absolute results. 


Relative results. 


Species and time of sowing. 


Ger- 
mina- 
tion. 


Death 
per hun- 
dred 
seed- 
lings. 


Final 
stand 
based 
on seed 
sown. 


Ger- 
minal 
tion. 


Death. 


Final 
stand. 


Finns anstriaca: 


Copper sulphate... 
None 

1 


} ' 


Perct. 
1 40.1 
\ 15.8 

/ 37. 5 
\ 19.5 

f 27.1 
\ 25.9 

/ 30.7 
\ 23.7 

/ 16.6 
\ 11.6 

f 37.5 
\ 14.2 


59.0 
91.0 

99.0 
lOC.O 

35.2 
62.3 

27.6 
69.0 

67.8 
67.0 

20.0 
66.0 


Perct. 

21.3 

1.3 

.5 


16.1 
7.6 

21.0 

5.8 

, 9.0 
6.3 

28.9 
6.4 


254 
100 

192 
100 

105 
100 

130 
100 

143 
100 

264 
100 


65 
100 

99 
100 

56 
100 

40 
100 

101 
100 

30 
100 


1,638 
100 


r.banksiana: 

Fall 


Zinc chlorid \ • , 

None / • 

Copper sulphate ... 1 „ 

None 1 ^ 

\ 


(a) 
(a) 

212 
100 




Zinc chlorid 


} » 


362 
100 




[Copper sulphate... 


143 
100 




Zinc chlorid 

None 






452 
100 




(Copper sulphate 
J and zinc chlorid. 
[None 




Average, fall and spring. . 


28.0 
18.9 

/ 46.1 
\ 40.9 

f 42.9 
\ 36.6 

/ 58.1 
\ 50.9 

f 61.3 
\ 52.7 


37.7 
66.1 

20.1 
59.8 

16.1 
57.4 

24.0 
45.1 

611.8 
6 39.1 


18.8 
6.5 

34.8 
16.0 

33.1 
18.1 

42.2 
28.7 

46.6 
31.0 


148 
100 


57 
100 


289 
100 


P. ponderosa: 


fCopper sulphate... 
None 


} ' 

} ' 
} » 
} ' 


113 
100 

117 
100 

114 
100 

116 
100 


34 
100 

28 
100 

53 
100 

30 
100 


217 
100 


Fall . 


Zinc chlorid 

None 






183 
100 




[Copper sulphate... 


147 
100 


Spring t 


1 Zinc chlorid 

[None 






100 


Average, fall and spring. . 


[Zinc chlorid and 
\ copper sulphate. 
[None 


52.1 
45.3 


18.0 

50.4 


39.2 
23.5 


115 
100 


36 
100 


167 
100 








- 



a No expression possible, b The damping-off percentage was determined in but fi\'e of the six tests. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 11 

Copper sulphate could probably have been used with success in 
quantities less than one-fourth ounce per square foot. A three-eighths 
ounce zinc-chlorid treatment proved unsuccessful. Both disinfect- 
ants were dissolved in water and applied in most cases just after seed 
sowing, as was done with acid. Both were found harmless to the 
dormant seed in these .concentrations, but both were capable of 
injuring germinating seedlings in just the same way as is described 
for acid at Halsey. This injury was prevented by extra watering 
during the germinating period, in the same way as acid injury was 
prevented at the Halsey and Kansas sand-hill nurseries. The 
amount of this extra watering needed at the Garden City nursery was 
very slight. 

An interesting thing that developed at this nursery, as well as at 
the Kansas sand-hill nursery, was the control of spring damping-off 
by treatments applied during November of the preceding year. Part 
of the fall-sown plats whose results were shown in Table II were 
treated at the time of sowing and part as soon as the soil thawed the 
following spring. Though the seedlings did not appear until the 
following April, disinfectants applied in November seemed to protect 
them from damping-off as well, or practically as well, as disinfectants 
applied in March. At this nursery and at the five preceding at which 
repeated tests were made, the treatments developed have been put 
into regular large-scale use by the nurserymen, with good results. 

RESULTS OF SOIL DISINFECTION AT NURSERIES WHERE TESTS HAVE BEEN LIMITED 

TO A SINGLE SEASON. 

At Porvenir, N. Mex., Dundee, 111., Lincohi, Nebr., the nursery of 
the State Board of Forestry in Vilas County, Wis., and the greenhouses 
of the United States Department of Agriculture at Washington, D. C, 
single-season tests of sulphuric acid have given good results. At all 
of these places no need was found for any special watering provision for 
the prevention of chemical injury from the acid. The tests at 
Dundee and Lincoln were made during an unusually rainy season. 
It may develop that in years with less rain more watering will be 
needed at these places. 

At four other nurseries where disinfectants were tested in this pre- 
liminary way the results were less definite. At MorrisviUe, Pa., 
ah-eady referred to, there was distinct evidence that a very weak acid 
treatment with sufficient watermg during the germinating period will 
be entirely successful in controUmg the small amount of damping-off 
which normally occurs there. One-eighth ounce of acid per square foot 
it is thought will be sufficient. Insufficient watering jDrevented the 
securing of exact information from the tests conducted. At Poca- 
tcUo, Idaho, a smgle test of acid on beds of Douglas fir had no con- 
spicuous effect on the amount of damping-off. As the soil is found 
to effervesce on the addition of acid, success with acid does not 



12 BULLETIN 453^ U. S. DEPARTMENT OF AGRICULTUEE. 

appear probable. At Cass Lake, Miim., acid was tested on seed beds 
of Norway pine (JPinus resinosa) during a period of weather so abnor- 
mally cold and wet that the seed lay in the ground for a number of 
weeks before germination was completed. Under these conditions, 
for the first time in the writers' experience, the acid treatment of the 
beds at the time of sowing resulted in decreased germination. No 
special watering was needed to prevent injury to the tips after the 
seedlings began to come up ; in fact, very frequent watering given one 
bed resulted in less germination and therefore poorer results than in 
acid beds with no extra watering. Despite the decrease in germina- 
tion, the acid beds, because of the almost complete control of damping- 
off by the treatment, finished the season with an average of 135 seed- 
lings for every 100 in the untreated plats, and the acid plats which 
were not given excessive water had 151 seedlings for every 100 
in the untreated plats. This fact and the fact that acid beds which 
had first been treated with lime and were thereby kept from injury to 
the seed produced 228 seedhngs for every 100 in the untreated plats 
indicate that in ordinary seasons the treatment with acid alone will 
be entirely successful. The use of lime with acid is usually undesirable 
because by neutralizing the acid it permits the parasites to resume 
work soon after treatment. Formaldehyde gave better results than 
acid at Cass Lake. While it is thought that this would not be the 
case in a normal season, it is evident that further tests are needed to 
determine what treatment to use. 

The danger in drawing conclusions from the results of three or 
four test plats is shown especially well by the results obtained by 
Prof. H. H. York, of Brown University, in testing treatments recom- 
mended by the writers. Treatments involving five-sixteenths ounce 
of sulphuric acid, five-eighth ounce of zinc chlorid, and 2^ ounces 
of cane sugar, respectively, per square foot of bed, resulted in final 
stands from 4^ to 15 times as dense as in most of the untreated plats. 
One untreated plat, however, gave a stand as dense as that on the 
treated plats. This throws doubt on the reliability of the results on 
the other plats and indicates that only repeated tests, each with 
plenty of untreated plats for abundant comparison, are reliable 
enough to be used as a basis for conclusions. 

At several of the nurseries mentioned above other soil treatments 
were tried with success. At Glen view. 111., where acid failed, copper- 
sulphate treatment resulted in doubling the final stand. The good 
results obtained on otherwise untreated soil at Dundee, lU., by the 
addition of cane sugar were rather surprising; indications of damp- 
ing-off control by sugar were also observed at Garden City, Kans., 
East Tawas, Mich., and Providence, K. I. Further tests are required. 
At East Tawas, a test of acid in an unfavorable season did not give 
as good results as desired, while formaldehyde resulted in a doubled 
stand. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 13 

RESULTS OF SOIL DISINFECTION IN GREENHOUSE TESTS. 

In the greenhouses of the Department of Agriculture at Washing- 
ton, D. C, disinfection of soil by steam pressure has proved reason- 
ably effective in controlling damping-off of pine seedhngs in most of a 
rather large number of tests conducted during the winter months. 
During the summer there is so much reinfection that the results of 
heat disinfection are uncertain. A single test of sulphui'ic acid in 
the spring of 1915 indicated the value of acid treatment in reducing 
damping-off in the greenhouse. Four flats of jack pine were treated 
with sulphuric acid in quantities of three-sixteenths to three-eighths 
ounce per square foot just after sowing. • The relative survival in the 
acid flats, taking the average survival in the untreated flats as 100, 
was 128. The relative survival in flats disinfected by steam was only 
105. Three-sixteenths ounce of acid per square foot gave as good 
results as heavier treatments. The soil used in this experiment was 
a mixture of seven parts loam, four parts sand, two parts manure, 
and one part leaf mold. 

RESULTS IN DISINFECTING THE SOIL OBTAINED BY OTHER INVESTIGATORS. 

Spaulding,* who appears to have been the originator of the use of 
sulphuric acid for soil disinfection, has tested acid and other disin- 
fectants at several places. In greenhouse tests formaldehyde and 
copper sulphate were found valuable, but the sulphate injured seed- 
lings. At Saranac Inn, N. Y., formaldehyde apparently increased 
damping-off one season and controlled it in a later season, while 
sulphuric acid injured seedhngs and gave results which from the 
economic standpoint were inconclusive. 

At Burlington, Vt., results with disinfectants in pine-seed beds have 
been reported by Spaulding,^ Jones,^ Gilford,* and Burns.^ Formal- 
dehyde appHed five days before sowing and allowed to evaporate for 
two days before sowing killed part of the seed. AU of the writers 
fomid formaldehyde of decided value in controlling damping-off. 
Jones and Spauldmg report a single test m which but one-tenth 
ounce (0.106 oimce) of formaldehyde was used per square foot and 
excellent restdts obtained. The subsequent tests reported have been 
with amomits of 0.48 ounce or over per square foot. The treatment 
used by Burns, of 0.58 omice of formaldehyde per square foot, applied 

1 Spaulding, Perley. The damping-off of coniferous seedlings. In Phytopathology, v. 4, no. 2, p. 73-88, 
2fig., pi. 6. 1914. 

The treatment of damping-off in coniferous seedlings. U. S. Dept. Agr., Bur. Plant Indus. Circ. 

4, 8 p. 1908. 

2 Spaulding, Perley, 1914. Op. cit. 

3 Jones, L. R. The damping-off of coniferous seedlings. In Vt. Agr. Exp. Sta. 20th Aim. Rpt., 1906-7, 
p. 342-347. 1908. 

* Gifford, C. M. The damping-off of coniferous seedlings. Vt. Agr. E.xp. Sta. Bui. 157, p. 141-171, 10 fig., 
4 pi. 1911. 

ii Bums, G. P. Studies in tolerance of New England forest trees. I. Development of white pine seedlings 

in nursery beds. Vt. Agr. Exp. Sta. Bui. 178, p. 125-144, 2 fig., 4 pi. 1914. 



14 BULLETIN 453, U. S, DEPARTMENT OF AGRICULTUEE. 

11 days before sowing, with tlic beds tightly covered during the 
interim, appears to have been entkely harmless and very effective 
against the disease. Burns fomid tlie sulphmic-acid treatment used 
by the writers at Halsey, three-sixteenths omice per square foot 
applied at time of sowing, as effective as the formaldehyde and non- 
injurious. Spaulding,^ reporting tests made m 1907 and 1908, also 
obtamed excellent results with acid, which was not tested by Jones 
and Gifford. He in addition obtained good results in preliminary 
tests with copper sulphate and with sidphur. 

Acid was also used at an unnamed locality by Gussow,^ with 
excellent results. 

Charcoal, an amendment rather than a disinfectant, is reported by 
Retan^ to be of value against damping-off in the clay soil at Mont 
Alto, Pa.j and to result in the increased size of pine seedlings. A layer 
3 inches deep was spaded into the soU. Annual treatment with this 
amomit would, of course, be unpracticable. However, Retan states 
that the effect of such charcoal addition is permanent. Further 
experience seems necessary to confirm the permanent value of char- 
coal for preventing damping-off on this and other sods. A smgie test 
by the writers of a lighter application of charcoal at the Cass Lake 
nursery proved unsuccessful. 

SOIL-DISINFECTION TESTS SUMMARIZED. 

The writers' experience with soil disuifectants at the different 
nurseries, as well as the experience of other experimenters, has been 
so varied that it is rather difficult to correlate the results. Correlation 
wdl be made easier by- reference to Table III, m which are summed 
up all of the successful tests of treatments of coniferous seed beds of 
which the writers have been able to learn. In the tests at Halsey, the 
Kansas sand hdls. Garden City, Lincoln, Cass Lake, Dmidee, Glen- 
view, and East Tawas, the experimental treatments were applied by 
the writers or their assistants, with the cooperation of those in 
charge of the nurseries. At Fort Bayard the tests were conducted by 
Mr. H. C. Tiu-ner, at Monument by Mr. W. H. Schrader, at Porvenir 
by Mr. H. D. BurraU, at Pocatello by Mr. Arthur P. Say, in VUas 
County, Wis., by Mr. W. D. Barnard, at Providence by Prof. H. H. 
York, at MorrisviQe mider the du-ection of Mr. John Foley, and at 
Haugan by forest officers, following recommendations made by the 
writers. 

1 Spaulding, Perley, 1914. Op. cit. 

2 Giissow, H. T. Diseases of forest trees. In Com. Conserv. Canada, Rpt. 1st Arm. Meeting, 1910, p. 
136-137. 1910. 

3 Retan, G. A. Charcoal as a means of solving some nm-seryi)roblems. /m Forestry Quait., v. 13, no. 1, 
p. 25-30. 1915. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 



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18 BULLETIN 453, U. S. DEPAETMENT OF AGRICULTURE. 

In most of the treatments listed in Table III the substance used 
was apphed just after the seed was sown and covered. Formaldehyde 
kills some of the seed if so used and was accordingly applied several 
days before seed sowmg. Either the soil was treated a week or more 
before sowing in the formaldehyde plats and kept covered with heavy 
paper for a few days after treatment, or else the treatment was made 
but two or three days before sowing and no paper was used. Even 
with these precautions part of the seed was killed in the tests at 
Burhngton, the Kansas sand hills, and Glenview. Zinc ohlorid killed 
the dormant jack-pine seed in two heavily treated plats at Garden 
City which were sown and treated in the fall and remained dormant 
throughout the winter. It ordinarily produces no such effect. Sul- 
phuric acid has also shown a tendency to decrease the germination 
percentage in two or three tests of fall treatments, but ordinarily has 
no such effect in spring-sown beds. It has never had more than a 
slight delaying effect on germination except durmg very wet, cold 
weather, and its harmful effect on dormant seed in sprmg-sown beds 
is negligible. Copper sulphate shows even less tendency than sul- 
phuric acid to injure dormant seed. Sulphuric acid, copper sulphate, 
and zmo chlorid are all inclined in some soils to injure the root tips 
of seedlings just after germination begins; on such soils the injury 
to seedlings can be prevented by very frequent watering duruig the 
germinating period. Column 7 of Table III indicates for the best 
strength of each successful treatment whether or not such extra 
watering is required. It will be noted that in formaldehyde beds 
extra watering is nowhere necessary. For the other three disinfec- 
tants it is more commonly necessary on very light sandy soils than on 
heavy soils. 

Air-slaked lime, powdered sulphur, and charcoal were apphed dry, 
and in the tests reported by the writers were raked into the upper 
3 inches of soil. Copper sulphate was applied in the form of dry 
powder in the tests reported by Spaulding.i Sulphuric acid, formal- 
dehyde, zinc chlorid, and in the tests reported by the writers copper 
sulphate were apphed in solution. The strength of all treatments 
is expressed in the number of ounces of substance per square foot of 
bed. For sulphuric acid and formaldehyde the fluid ounce (29.574 
o. c.) is the unit of measure, while for all other substances the avoir- 
dupois ounce (28.35 gm.) was used. Three-fourths of an ounce per 
square foot is practically equivalent to 1 ton per acre. The amount 
of water used in dissolving the dismfectants varies from 1 to 2^ pints 
per square foot. For a soil already wet 1 pint is sufficient; for a dry 
fine-textured soil 2^ pints may be needed to secure a proper distri- 
bution of the disinfectant. Ordinarily, 1§ or If pints per square 
foot are satisfactory. 

1 Spaulding, Perley. The treatment of damping-off in coniferous seedlings. U. S. Dept. Agr., Bur. 
Plant Indus. Cire. 4, 8 p. 1908. 



DAMPING-OFF OP CONIFEROUS SEEDLINGS. 19 

COST OF DISINFECTANT TREATMENTS^ 

Because of the differenoes between the nurseries where tests have 
been made, both m the details of the treatment used and in the scale 
of the operations and the equipment of the nursery in such matters 
as water supply, it is impossible to make any statement as to the 
exact costs of treatment. 

At two of the nurseries (Fort Bayard, N. Mex., and Haugan, 
Mont.) mentioned in the foregoing pages, at which the treatments 
have been in use long enough to permit a definite statement, all that 
is needed is an application of acid solution to the beds just after 
sowing. The treated beds thereafter require no more attention than 
do those untreated. At six other nurseries, where the acid has 
been in use for a less time, the same appears to be true, while for a 
seventh nursery, where copper sulphate gives the best results, the 
same simple method of application appears satisfactory. For these 
nine nurseries, then, the treatment which has so far appeared most 
effective involves no expense except that of the single disinfectant 
application. Different amounts of disinfectant are required for 
different soils, and because of the rather high cost of transporting 
sulphuric acid the price of that disinfectant varies greatly in different 
locahties, so that exact costs of the treatment can not be given. It 
is very evident, however, that both the sulphuric-acid and the 
copper-sulphate treatments applied in this way are decidedly cheaper 
than either the formaldehyde or heat disinfection methods as used 
in out-of-door seed beds of tobacco and other truck cx'ops. At 
many raihoad centers sulphuric acid can be obt;ained in carboy lots 
for 2 J cents per pound ^ or even less, while copper sulphate should not 
ordinarily exceed 8 cents per pomid.* With copper sulphate one- 
fourth avoirdupois omice and sulphiuic acid one-fourth fluid ounce 
per square foot will be sufficient or more than sufficient at most 
nurseries. Judging from the experience of truck-crop experimenters 
and from the results of the writers and others with formaldehyde at 
several nurseries, one-fourth to one-half fluid ounce per square foot 
of the more expensive formaldehyde will be needed on most soils to 
secm-e sufficient disinfection. If it develops that it is necessary to 
use the tight cover prescribed by most writei-s to prevent premature 
evaporation and to treat additional soil for use in covering the seed 
after sowing, these items will further increase the expense of for- 
maldehyde treatment. The expense of steam disinfection of out-of- 
door seed beds of tobacco is given by Johnson ^ as $7.50 to $10 per 
thousand square feet of bed. Tliis is a much higher cost than is to 
be anticipated with acid or copper-sulphate treatment. 

1 These prices prevailed prior to the outbreak of the European war. It is assumed that the present 
higher prices are temporary. 

2 Johnson, James. The control of diseases and Insects of tobacco. Wis. Agr. Exp. Sta. Bui. 237, p. 10, 
1914. 



20 BULLETIN 453, U. S. DEPARTMENT OF AGRICULTURE. 

Sugar, at first glance an excessively expensive amendment for use 
on soil, is not so much more expensive than formaldehyde in the 
quantities which appear of value in preventing damping-off at three 
of the, nurseries. If some unrefined sugar-bearing substance could 
be substituted for the table product used in the experiments, it is 
entirely possible that for certain soils sugar would become an eco- 
nomically satisfactory treatment for coniferous seed beds. Zinc 
chlorid also, while probably somewhat more expensive than formal- 
dehyde, is not so costly as to be economically impossible on soils 
where it may prove to be the most effective disinfectant. Both of 
these substances are, however, decidedly more expensive than sul- 
phuric acid or copper sulphate. 

The most troublesome thing about the treatment with acid is that 
at some nurseries it is necessary to give treated beds frequent water- 
ings during the germinating period for the protection of the roots of 
the seedhngs against chemical injury. This has been found to be 
the case in acid plats at three nurseries, and with copper and zinc 
salts at one nursery, respectively, where these disinfectants have 
given evidence of greatest commercial value. At one of these 
nurseries (Halsey, Nebr.) figures are available which indicate the 
cost of the treatment with acid and subsequent watering. The 
additional labor cost of the seed sowing, due to the use with it of the 
acid apphcation, was furnished by the forest officers. The figures 
follow. 

Additional cost of seedbeds at Halsey, Nebr., due to acid treatment, per 1,000 square feet 
(space occupied by paths excluded). 

Cost of acid (allowing for freight, drayage, and waste) 11. 00 

Additional labor in seed sowing, due to application of acid 80 

Labor cost of extra waterings required by acid beds during germinating period. 2. 70 
Depreciation of containers for applying acid solution, and increased supervisory 
cost (estimated), not to exceed 50 

Total charge against treatment 5. 00 

The above figm^es were obtained in 1912. Subsequent changes in 
nursery practice have probably changed the costs, but no later fig- 
ures aie available. 

This cost of $5 per 1,000 square feet, or one-half cent per square 
foot of seed bod, would seem very high if put on an acreage basis. 
For extensive farming operations it would, of com-se, be excessive.. 
But because of the very high cost of coniferous seed and the very small 
area on which the seed-bed operations of even a large nm'sery are 
usually concentrated, this cost is easily ofi^sot by even a sfight increase 
in the stand of seedlings secured. The actual cost of production of 
seedlings is seldom less than 50 cents per thousand by the end of the 
first growing season. The prices commonly quoted by commercial 
nurseries for seedlings of most species which have remained in the 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 21 

seed bed for a second year run from foiu- to ten times this figure. If 
the minimum stand commonly desu-ed, i. e., 100 seedhngs per square 
foot, is obtained on untreated beds, an increase in stand of only 20 
per cent will mean an increased production of 20,000 seedlings, of a 
minimum value of $10 for the 1,000 square feet taken as a unit. 
Even in this minimum case, therefore, the treatment may be con- 
sidered to have paid for itself twice over. The ordinary net results 
should be still more favorable, for the following reasons: 

(1) At nurseries where acid or copper sulphate can be used without especially 
frequent watering the cost of the treatment will be less than at Halsey. At only 
one other nursery has it been found necessary to water the treated beds as much as 
at Halsey. 

(2) The average increase in stand resulting from the treatment is much gi-eater 
than the 20 per cent used in the preceding calculations. Figures based on exact 
counts for two or more seasons are available from five localities (Halsey, the Kansas 
sand liills, Garden City, Fort Bayard, and Monument). The average increase in stand 
resulting from the best treatments at all of these nurseries is exactly 100 per cent. 

(3) An additional advantage, of greater value than the average increase in stand, 
is the stabilization of output through the j^revention of damping-off epidemics. 

(4) A further advantage is the development of more uniform stands in the beds by 
the prevention of damping-off patches. 

(5) In addition to the advantages from increased stands and decreased damping-off, 
disinfectant treatment often has incidental results in the way of weed control or of 
increased growth of the conifers wliich may by themselves more than pay all costs 
connected \\dth the treatment. These secondary advantages are discussed in the 
following paragraphs. 

SECONDARY ADVANTAGES FROM DISINFECTANT TREATMENTS. 

STIMULATION OF GERMINATION CAUSED BY DISINFECTANTS. 

It is common and, in fact, almost invariable with all of the disinfect- 
ant treatments which have been successfully used to find a higher ger- 
mination percentage in the treated plats than in the controls. In 
some cases this increase in germination (or, more accurately speak- 
ing, in the number of seedlings which appear above the soil surface) is 
very large. Whole groups of treated plats have in some cases given 
an apparent germination percentage three times that prevailing in 
the untreated plats in the same group. While such large increases 
are not common, it is a very frequent thing for the advantage in 
increased germination to be greater than that resulting from the 
control of damping-off after germination. (Compare the first and 
second columns under the head of ''Relative results" in Tables I 
and II.) Such increases are mainly due to the control of parasites 
by the treatments; in the untreated plats Pythium and Rhizoctonia 
kill many seedlings or sprouting seeds before they break through the 
soil cover. However, in some cases at least, disinfectants, by direct 
action or by their effect on the soil, appear to cause seed to germinate 
which would otherwise have remained dormant. The advantage 
from this apparent stimulation of dormant seed can not be quanti- 



22 BULLETIN 453, U. S. DEPARTMENT OF AGRICULTURE. 

tatively separated from the advantage due to the control of parasites. 
The sum of the results of the treatments in stimulating germination, 
protecting the germinating seed from parasites, and in preventing 
damping-off are all cumulatively expressed by the increase in final 
stand in the treated over the untreated plats at the end of the season. 

INCREASED SIZE OF SEEDLINGS CAUSED BY DISINFECTANTS. 

At three of the nurseries where experiments were conducted for 
several seasons it was found that there was a distinct increase in the 
size of pine seedlings as a result of treatment with sulphuric acid. 
Immediately after the seedlings come up, the only effect, when there 
is a noticeable effect, is a decreased growth rate in the acid plats. 
This effect later disappears, and during the latter part of the season 
the seedlings normally grow faster in the acid plats than in the con- 
trols or in plats treated with most other disinfectants. At the nursery 
at Halsey during 1912 and 1913, seedlings were measured in five 
experimental series, three of jack pine, one of western yellow pine, 
and one of Norway pine. The average of all five shows that the acid 
caused an increase in height of 37 per cent. 

At the nurseries in the Kansas sand hills and at Garden City acid 
had a much more surprising effect. The results with western yellow 
pine indicated a moderate increase in the first season's growth, in 
agreement with the Halsey results, this increase being still distinctly 
noticeable, at least at the sand-hill nursery, at the end of the second 
year's growth. But with jack pine at these nurseries unexpectedly 
large increases were secured. At both nurseries jack pine ordinarily 
makes a slow growth during the first season. In plats sown in Novem- 
ber, 1912, and treated with sulphuric acid early the following spring, 
the average height of jack pine after a year's growth was found to be 
more than three times that of the seedlings in the untreated plats. 
The effect of acid treatment on the seedlings at the Garden City 
nursery is shown in Plate II, figure 1. At the Kansas sand-hill nurs- 
ery, root systems as well as tops were examined. The increase in 
root development appeared entirely commensurate with the increased 
size of the tops. The air-dry weight of the tops at the sand-hill 
nursery was 7^ times as great in these acid plats as in the untreated 
plats. 

It is not to be expected that the acid will result in increased growth 
of any species in all soils, as the effect on growth is apparently due 
not to direct stimulation of the conifers, but rather to the effect of 
the acid on the soil. It is interesting to note that the only great 
increases in growth have been found at the nurseries where the soil 
water contained the largest quantities of acid carbonates. 

The economic value of a moderate increase in growth, such as 
is secured at Halsey, depends largely on the system of handling stock. 
If the seedlings are to be held in the seed bed for two years an increase 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 23 

in the first season's growtli is not very important. If, however, it is 
desired to transplant the stock at the end of the first season the 
increase in size due to acid, at least in the jack pine, which is ordi- 
narily most helped, may mean the difference between success and 
failure. In a case of great increase in size, such as was secured by 
the acid treatment of jack pine at the two, nurseries in Kansas, the 
economic results are more positive. It was entirely impossible to 
raise 1 -year-old stock large enough to transplant without the use of 
acid. With the acid treatment 1 -year-old seedlings were produced 
which appeared to be in every way the equal of mitreated 2-year-old 
stock for transplanting purposes. The time and expense involved in 
holding the stock for a second year in the seed beds were avoided. 
At the Garden City nursery the advantage went even farther than that. 
The seedlmgs in the untreated beds were so small that they could 
not withstand winterkiUmg and were practically all killed before the 
second growing season. The seedlings in the acid plats immediately 
adjacent came through the winter practically without loss. While, 
as already stated, the acid proved ineffective at this nursery so far as 
the control of damping-off is concerned, on account of the carbonates 
present in the soil, its effect on growth and the resultant freedom from 
winter loss alone positively indicated its use on jack-pine beds in 
combination with the toxic salts found most valuable for damping-off 
control. 

WEED CONTROL BY DISINFECTANTS. 

At the nurseries where weeds are troublesome in the seed beds the 
effect of the disinfectants on the weeds is probably the most important 
of the secondary results of the treatment. The plants commonly 
occurring as weeds seem on the whole much more sensitive to acids 
and to copper and zinc salts than do the conifers. The result is that 
treatments so worked out as to be entirely harmless to the coniferous 
seedlings in the beds are found at a number of nurseries to keep the 
beds almost entirely free from weeds during the first three or four 
weeks after the germmation of the conifers. This is just the time 
when most weeds can be expected to appear in the mitreated beds 
and when the delicacy of the yomig conifers makes it difficult to do 
even hand weeding without breaking or puUing up many of the seed- 
lings. The economic value of this weed-control feature varies with 
different nurseries. At some places the efficiency with which weeds 
are controlled is less than at others. At some places weeds are not 
numerous enough to make weed control a consideration of any very 
great importance. The only place at which an attempt has been 
made to reduce to a dollars-and-cents basis the value of the weed- 
controUing effect of a damping-off control treatment is at Halsey, 
Nebr. Approximate figures obtained from forest officers indicate that 
the entire cost of the hand weeding required during the seasons consid- 



24 BULLETIN 453, U. S. DEPARTMENT OF AGEICULTURE. 

ered was approximately $7.60 per 1 ,000 square feet ; that for the season 
of 1912 the cost m the acid beds was $5.40 less than in the untreated 
beds; and m 1913, $5.20 less. This saving m cost of weeding exceeds 
the entire cost of the acid treatment as estimated during the first of 
these years. While the net value of weed control will not be as great 
as this m all seasons or at all nurseries, it appears that at several of 
the nurseries where tests have been carried on the benefit from weed 
control (PI. II, fig. 2) or from increased growth of the seedlings, or 
from both of these effects combined, will be sufficient to more than 
justify the treatment. In such cases the control of damping-off 
secured by the treatments will be- clear gain. It is especially inter- 
esting to note that in the seed beds of the Feather River Forest 
Experiment Station (California) a light sulphuric-acid treatment is in 
regular "u^e simply on account of its value as a weed killer, entirely 
irrespective of any effect on damping-off. 

CONCLUSIONS AS TO SOIL DISINFECTANTS. 

The results listed in Table III indicate the complexity of the 
problem of soil treatment to control dampmg-off parasites. It is 
at once evident that a single season's experiments conducted on a 
single soU do not fuj-nish a basis on which it is safe to make recom- 
mendations for general use. It furthermore appears that even after 
as extensive experhnents as are here reported, it is not possible to 
prescribe any one treatment which wdl be safe and eff'ective at aU 
nurseries. Heat, the disinfecting agent most commonly employed 
by workers with truck soils, has proved mferior to other methods at all 
of the nm-series at which it has been tried. Sulphur, which is re- 
ported to have given excellent results against certain root diseases of 
other plants, has on the whole given disappointing results in pine 
seed beds. Formaldehyde, sulphuric acid, copper sulphate, and zinc 
chlorid, the four most generally satisfactory substances, have each 
failed at one or more nurseries. Especially at nurseries where no 
adequate watering system has been installed the frequent watering 
which beds treated with acid or with copper and zinc salts sometimes 
require means considerable trouble and expense. 

However, after all these difficulties and drawbacks have been con- 
sidered, analysis of the results indicates that it will be practicable to 
control dampmg-off by some soil treatment at any nursery where it 
is troublesome. The first part of Table III, containing results at the 
seven nurseries where repeated tests have been made, is, of course, 
the most important. At all of these nurseries soil treatment has 
proved successful. It is significant that at aU but one of these seven 
sulphuric acid has proved successful. The acid failed at the seventh 
nursery only because of the high carbonate content of the soil, a con- 
dition rarely found at coniferous nurseries. At this seventh nursery 



DAMPING-OFP or CONIFEROUS SEEDLINGS. 25 

the cheap and siiuplc copper-sulphate treatment proved entirely sat- 
isfactory, with the alternative of zinc chlorid in case the copper salt 
should ultimately prove in any way unsatisfactory. At four of the 
seven, all that appears necessary to be done is to apply the disin- 
fectant, and thereafter let the beds take care of themselves. Al- 
though specially frequent watering is required during the germinating 
season by treated beds at the other three nurseries, the cost of this, 
as has been shown for Halsey, is not excessive, and the amount of 
extra watering required to protect the germinating seedlings at the 
remaining two nurseries is less than at Halsey. 

At the localities considered in the last part of the table, where the 
tests have been mainly or entirely limited to a single season, some 
facts of interest have been developed, though at none of them have 
the best control methods been definitely determined. At Dundee 
and Cass Lake a sufficient number of independent series of tests were 
made to show without doubt that some one of the treatments which 
resulted well will be found satisfactory for regular use. At most of 
the other nurseries in this second division the tests have been 
preluninary in character and are only indicative. At 13 of the 
14 localities listed in the second part of the table, the results of 
the tests indicated the value of one or more of the treatments 
tested. At 9 of these 13 the indications were that the disease 
could be controlled by a treatment which would not make neces- 
sary any special watering of the beds. At the one nursery where no 
results were obtained (Pocatello) sulphm-ic acid was the only substance 
tested under proper watering conditions, and its failure is easily ex- 
plained by the presence of soil carbonates. 

Considering separately the soil treatments employed, it appears 
that there are four reasonably promismg disinfectants for use on 
coniferous seed beds. The writers have experimented with approxi- 
mately 50 different sub'stances, alone or in combination, in the 
treatment of seed beds, and these tests, in conjunction with the re- 
sults of the writers and of others which are summarized in Table III, 
indicate that the most generally satisfactory substances for such use 
are sulphuric acid, copper sulphate, zinc chlorid, and formaldehyde, 
of which the acid is the most promising. 

The use of sulphuric acid has been reported at 20 different 
localities. At 17 of these it has given indications of value. Its 
failure at Pocatello and Garden City is merely an indication that 
it will not succeed in the presence of large quantities of soil car- 
bonates. At Glenview, 111., the tests were very preliminary and 
therefore not sufficient to show that it was valueless. It has become 
established as a regular part of nursery practice at all nurseries where 
it has been repeatedly tested, with the exception noted of the one 
where the soU contained carbonates. At several nurseries m the 



26 BULLETIN 453, U. S. DEPARTMENT OF AGRICULTURE, 

Middle West, where the acid in single-season tests did not give as 
good results as other substances, there is reason to believe that the 
results of the acid treatment will be much better in normal seasons. 
The acid has to recommend it, in addition to its effectiveness against 
damping-off, the elements of cheapness, efficiency in killing weeds, 
and in many places of causing an increased growth of the conifers. 

Copper sulphate, tested at 11 different places, gave no indica- 
tions of value at three of them and very doubtful indications at the 
fourth. At the others it gave more or less indication of value, reaching 
the stage of regular nursery use at one nursery and at two others giving 
better results than the acid in single-season tests. Copper-sulphate 
treatment costs about the same as treatment with acid and is nearly 
or quite as effective in killing weeds, but has not been observed to 
result in any marked increase in growth of the conifers. 

Data as to the value of zinc chlorid were secured from ten nur- 
series. At one of these it failed. At three others the tests were very 
preliminary, and while value was indicated the extent of value was 
not determined. At four it indicated value, but to a less extent 
than other treatments used. At the remaining two it indicated 
sufficient value to rank as a successful treatment. At both of the 
nurseries where it has been given repeated tests it has proved as 
good as any other treatment. At both of these, however, it has not 
been shown to be decidedly better than cheaper treatments (copper 
sulphate at Garden City and sulphuric acid at Fort Bayard), and 
therefore is not likely to replace them in regular use. Like copper 
sulphate, it kills weeds in addition to decreasing damping-off and 
has not been observed to cause increased growth of the conifers. 
The relatively high cost of this disinfectant, approximately one-half 
cent per square foot on heavy soils for material alone, is a point 
against it. Both copper sulphate and zinc chlorid cause injury to 
pines on some soils where acid seems entirely harmless. This means 
that their use will at most places necessitate more watering during 
the germinating period than is necessary when acid is used. 

Data as to the value of formaldehyde in controlling damping-off 
have been secured from 12 places. At two of these it has failed. 
At another it has indicated value; extent undetermined. At five 
others it has indicated less value than sulphuric acid, while at the 
four remaining places it has been reported as successful.^ It also 
kills weeds, has little effect on the growth of pnies, and has the 
advantage of never making necessary any extra watering in treated 
plats. The points against it, in addition to its inferior efficiency in 
controUmg damping-off at many nurseries, are its high cost and its 
tendency to kill seed if applied at or near the time of sowing. It is 

1 At Dundee, formaldehyde alone was considered a failure, as the only successful formaldehyde plats 
were those also treated with zinc chlorid. 



DAMPING-OFF OP CONIFEROUS SEEDLINGS. 27 

necessary to treat beds some time before sowing, this period varying 
from two days to two weeks in different soils. The generally pre- 
scribed practice of covering the beds to prevent reinfection or pre- 
mature evaporation of the formaldehyde during this period is 
troublesome and expensive. Further tests may prove it unnecessary. 

In considering disinfectants, the idtimate as well as the immedi- 
ate results should be taken into account. No data are at hand to 
show the effects of repeated treatments of soil with toxic salts or 
formaldehyde. The effects of single treatments seem to be purely 
temporary. Treatments of one-eighth and three-sixteenths ounce 
of copper sulphate per square foot (equivalent to 341 and 511 pounds 
per acre, respectively), which are expected to be quite sufficient for 
use against damping-off at most nurseries, involve additions of only 
one-sixth and one-fourth, respectively, of the amount which has 
been used in single treatments at Garden City without any notice- 
able permanent effect. Sulphuric acid might ultimately bring about 
an acid condition of the soil if used repeatedly, but this could be 
easily remedied at any time by the addition of hme. The only 
possible bad effect of contmued use of the acid would therefore be 
an accumulation of sulphates. The sulphur added in a treatment of 
three-sixteenths fluid ounce of acid per square foot would be, roughly, 
300 pounds per acre. If repeated every year, this would, of course, 
mean a considerable change in sulphur content. However, it would 
be a very rare thing for a treatment to be appHed two consecutive 
years on the same soil, and where rotation is practiced five or six 
years commonly elapse between the growing of two crops of seed- 
lings of susceptible species on the same soil. This minimizes the 
likelihood of any bad cumulative effect by the treatments. 

The comparison of the four disinfectants considered in the fore- 
going paragraphs does not make possible a final statement as to 
their relative value. At most of the localities listed in Table III there 
have not been enough tests to give conclusive results. It is believed 
from the somewhat incomplete evidence secured that at most nur- 
series soil treatment vnth sulphuric acid wiU be found a satisfactory 
and probably the most satisfactory means of decreasing damping- 
off and that where it is not satisfactory success can be secured with 
some one of the other disinfectants — copper sulphate, zinc chlorid, 
or formaldehyde. 

It is, of course, recognized that the treatments so far devised are 
not as simple and effective as are desired. Further tests of these 
disinfectants and of numerous others are under way. The problem 
of damj)ing-off control is also being attacked from other directions 
than that of simple soil dismfection. It is hoped that a single disin- 
fection method may be found which can be used on any soil and which 
will not require any unusual precautions against chemical injury; or, 
failing this, that some less direct procedure against the parasites 



28 BULLETIN 453, IT. S. DEPARTMENT OF AGRICULTUEE. 

will be found satisfactory. The securing of final results along these 
lines will of necessity require several years' experimenting, because 
of the differences between different soils and the number of species 
of parasites and of conifers which have to be considered. Meantime, 
the foregoing results are published to enable nurserymen to make use 
of the experience already gained. Under the following heading are 
outlined a number of treatments, at least one of which should be 
found successful and profitable at any coniferous nursery where 
damping-off is prevalent. 

SOIL TREATMENTS RECOMMENDED. 

No treatments can be guaranteed as either safe or effective on all 
soils. However, it is nearly certain that for any nursery at least one 
of the following will be found successful. It is therefore recom- 
mended that any nurseryman who has serious losses from damping- 
off test some of these treatments on a small scale until he finds which 
one is best suited to his conditions. At least two successful tests 
should be made before a treatment is judged safe for large-scale use. 

Treatments for heavy soils. 

1. Sulphuric acid, three-sixteenths fluid ounce per square foot of bed, dissolved in 
from 1 to 2 pints of water per square foot of bed and applied immediately after the 
seed is sown and covered. 

2. Same as treatment 1, but use one-fourth ounce acid per square foot. 

3. Copper sulphate, one-eighth avoirdupois ounce per square foot, dissolved in water 
and applied immediately after the seed is sown and covered. 

4. Same as treatment 3, but use three-sixteenths ounce per square foot. 

5. Zinc chlorid, one-half ounce per square foot, dissolved in water and applied 
immediately after the seed is sown and covered. 

6. Formaldehyde, one-half fluid ounce per square foot, dissolved in water and 
applied 10 days before the seed is sown. Keep the bed covered with paper, tarpaulin, 
or tight shade frames during these 10 days. Do not spade up formaldehyde beds after 
treatment. If necessary to cover the seed with soil from outside of the plat, which 
has not been treated, use subsoil just dug up from at least 1 foot below the surface. 

7. Same as treatment 6, but apply the treatment only three days before sowing and 
do not cover with paper. 

8. Air-slaked lime, one-half avoirdupois ounce per square foot, applied dry and 
raked into upper 3 inches of soil just before sowing. (If hydrated lime is used, three- 
eighths ounce per square foot will be sufficient.) Immediately after seed is sown and 
covered apply three-tenths ounce of sulphuric acid per square foot, dissolved in water. 

9. Same as treatment 3, but use one-fourth ounce per square foot. 

In dissolving disinfectants, use sufficient water to make from 1 to 2 
pints of solution per square foot. Two pints should ])e used if the 
soil is dry; 1 pint is sufficient when the soil is wet. 

Treatments for sandy soils. 

1. Sulphuric acid, one-eighth fluid ounce per square foot, dissolved in water and 
applied immediately after the seed is sown and covered. 

2. Same as treatment 1, but three-sixteenths ounce per square foot. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 29 

3. Copper sulphate, one-tenth avoirdupois ounce per square foot, dissolved in water 
and applied immediately after the seed is sown and covered. 

4. Same as treatment 3, but one-sixth ounce per square foot. 

5. Zinc chlorid, five-sixteenths avoirdupois ounce per square foot, dissolved in water 
and applied immediately after the seed is sown and covered. 

6. Formaldehyde, five-sixteenths fluid ounce per square foot, dissolved in water and 
applied 10 days before seed is sown. Keep the bed covered with paper or tarpaulin 
during these 10 days. Do not spade up formaldehyde beds after treatment. If 
necessary to cover the seed with soil from outside of the plat, use subsoil just dug up 
from at least 1 foot below the surface. 

7. Same as treatment C, but apply only three days before seed sowing and do not 
cover beds with paper. 

8. Air-slaked lime, three-eighths avoirdupois ounce per square foot, applied dry 
and raked into the upper 3 inches of soil Just before sowing. (If hydrated lime is 
used, three-tenths ounce per square foot will be sufficient.) Immediately after the 
seed is sown and covered, apply one-fifth ounce of sulphuric acid per square foot, 
dissolved in water. 

In dissolving disinfectants for sandy soils, use sufficient water to 
make from 1 to 1^ pints of solution per square foot. One and one- 
half pints should be used if the soil is dry; 1 pint is sufficient if the 
soil is already wet. 

GENERAL DIRECTIONS FOR TREATMENTS. 

Commercial or tecluiical grades of all the disinfectants are satisfac- 
tory. The sulphuric acid purchased should be concentrated, having 
a specific gravity of at least 1.82, while the formaldehyde solution, or 
"formahn" as it is sometimes called, should be the strongest obtain- 
able, so-called 40 per cent solution, containing 37 per cent by weight. 
All disinfectants should be kept from the air as much as possible, as 
they change in strength if exposed. Copper sulphate requires the 
least care in this respect. Acid should he dissolved hy pouring it into 
the ivater — never hy the reverse process. Copper sulphate is quickest 
dissolved by putting it in a burlap sack and hanging it in the water 
just below the surface. Both of these solutions are corrosive to metal 
and should be handled only in wooden or earthenware containers and 
appKed with sprinklers which have been coated inside with hot 
paraffin.^ Acid is also hard on the hands and clothes. Men who use 
it on a large scale are very careful to keep their shoes heavily greased. 
Canvas gloves treated with hot paraffin or with a mixture of paraffin 
and a lighter grease, such as vasefine or lard, should be a valuable pro- 
tection for the hands. The charring of wooden containers used in 
making up dilute acid solutions probably can be largely prevented by 
washing them out well with water containing washing soda before they 

1 To coat a sprinkler with paraffin, get the cake paraffin sold hy grocerymen for use in sealing jelly and 
preserves. Shave off some of the paraffin into the sprinkler, and heat the sprinkler till all the paraffin is 
melted. Then turn the sprinkler around so that the liquid paraffin runs over the entire inside surface 
and finally pour what is left of the paraffin out through the spout. The whole can should be hot during 
the process, so that the paraffin will leave a thin coating; a thick coating cracks off too easily. The holes 
in the sprinkler head will have to be cleaned out with a pin or toothpick after the paraffin has hardened. 



30 



BULLETIN 453^ U. S. DEPAETMENT OF AGRICULTURE. 



Not 
treated. 



Treatment 
No. 1. 



Treatment 
No. 2. 



' Not 
treated. 



are put away at the close of work. A coating of linseed oil, very 
thoroughly dried, should also prevent the charring of wood. Acid 
which gets on the hands does no harm if washed off at once with water. 
In applying treatments on a large scale the sprinklers used should 
have the holes in the rose or sprinkler head enlarged, to permit 
faster worlv. 

In trying out the treatments in the foregoing lists, Nos. 1 and 2, 
the most promising, should be tested first. If both fail, the other 
treatments should be tried. If the acid solutions 
cause the soil to effervesce, some of the other treat- 
ments should be "tried at once, as on such a soil the 
acid is likely to fail. 

The treatments should be applied to small plats 
which are intermingled with untreated plats for com- 
parison. For test plats, 3 by 4 feet has been found a 
convenient size. Good results can be obtained with 
even smaller areas than tliis, but all plats should be 
at least 2 feet wide. An arrangement of plats wMch 
the writers have found satisfactory is shown in figure 1. 
The seed for each plat, treated and untreated, should 
be measured out separately, so that aU plats will get 
equal quantities of seed. Tlien the number of seed- 
hngs living on each plat at the end of the season will 
show wliich treatment is most valuable. In watering, 
shading, and in every other way treated plats should 
be handled just like the untreated beds, to make the 
test a fair one. 

As soon as the seedlings begin to drop their seed 
coats, plats treated with acid, copper sulphate, or zinc 
chlorid should be examined for chemical injury to the 
roots of the seedlings.^ If such chemical injury is 
found to occur, two courses will be open: (1) To test 
the same treatment again, watering the treated .plats 
thoroughly every day from sowing till germination 
is complete ; or, (2) to abandon the treatment which 
caused injury and try to get suflicient control of the 
disease by a weaker treatment or by another dis- 
infectant. Treatments 6, 7, and 8 are inserted especially for use at 
places where acid causes injury and where frequent watering is not 
practicable; they are reasonably certain not to cause injury to seed- 
hngs under any circumstances. 

1 The method of detecting chemical injury and illustrations of injured and uninjured seedlings have 
been given in a previous publicat ion: Hartley, Carl. Injury by disinfectants to seeds and roots in sandy 
soils. U. S. Dept. Agr. Bui. 169, p. 9, pi. 1. 1915. This bulletin can be obtained by sending 5 cents m 
coin to the Superintendent of Documents, Government Printing Oflace, Washington, D. C. 



Treatment 
No. 3. 



i 



Treatment 
No. 4. 



T 



Not 
treated. 



< 4'-- > 

Fig. 1.— Suggested 
arrangement of test 
plats. 



DAMPING-OFF OF CONIFEROUS SEEDLINGS. 31 

SUMMARY. 

(1) By damping-off is meant the killing of very young seedlings hy 
parasitic fungi. It is the most serious difficulty encountered in rais- 
ing coniferous seedlings. 

(2) To decrease losses from the disease excessive moisture and 
shade should be avoided. Caution must be used in following this 
recommendation or many seedlings may be killed by drought or by 
white-spot injury to the base of the stem. Damping-off can often 
be decreased by putting beds on very sandy soil. Seed should not 
be sown any thicker than necessary. It appears better to sow 
broadcast than in drills. Late fall sowing results in decreased losses 
at some niirseries and is worth trial. Proper attention to aU of these 
measures will decrease the losses from damping-off, but at most 
nurseries they are not sufficient really to control the disease. 

(3) The addition of lune, wood ashes, and in some cases nitrog- 
enous fertilizers seems to increase damping-off. Soil alkalinity 
appears to favor the disease. No effect has been noted from green 
manures. The use of unrotted stable manure has had very bad 
results; properly rotted manures seem less objectionable. Tankage, 
charcoal, and cane ^ugar are the only nonchsinfectant substances 
which to date have given any hope of disease control. 

(4) Soil disinfection has so far proved the best method of combat- 
ing damping-off. Of many methods tested, treatments with sul- 
phuric acid, copper sulphate, zinc chlorid, and formaldehyde have 
proved the most satisfactory. The disinfectants behave quite differ- 
ently at different nurseries. The results of treatments at many 
different localities are summarized in Table III. The acid has on 
the whole given the best results. Heat disinfection has been only 
partly effective. Disinfection by acid or copper sulphate is cheaper 
than by the other methods commonly recommended. 

(5) In addition to decreasing damping-off after the seedlings come 
up, the chemical disinfectants above mentioned, when piopeily used, 
cause an increase in the apparent germination and are very helpful in 
contiolling weeds. This latter effect alone at some nurseries pays 
the entne expense of the treatment. Sulphuric acid has, further- 
more, at some places resulted in marked increases in the late-season 
growth of pines. (See PI. II.) 

(6) In some soils formaldehyde kills dormant seed, and the other 
three most satisfactory disinfectants at some nurseries kill the root 
tips of germinating seedlings. By proper precaution, all such injury 
may be prevented. 

(7) The results obtained to date show that it is entirely possible 
and practicable to control damping-off by soil disinfection. Unfortu- 
nately, the varying behavior of disinfectants at different places. 



LIBRARY OF CONGRESS 



015 793 623 2 ^ 

32 BULLETIN 453, U. S, DEPARTMENT OF AGRICULTURE. 

renders it impossible to i*ecommend any single treatment which will 
be everywhere successful. Du*ections are given for simple tests, by 
which it is believed that any nurseryman can develop a treatment 
that will succeed under his conditions. 

(8) Further experiments are being conducted, with the hope of 
discovering either a more generally applicable disinfectant method 
than any of those so far tested or else a means of controlling the 
disease without the use of disinfectants. 



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